A power semiconductor is a semiconductor used for controlling and/or supplying electric energy, for example, converting AC to DC, reducing voltage, etc. A power semiconductor deals with a large amount of current, as compared to an LSI, etc.
Power semiconductors are extensively used. Examples include controlling a variety of motors, power converter and uninterruptible power source. A power semiconductor device (power semiconductor module), in which a power semiconductor switching die, such as an IGBT (Insulated Gate Bipolar Transistor), GTO and power transistor is packaged, is known. These power semiconductor devices are applied to various inverter devices depending on their breakdown voltage and/or current capacity. To increase a capacity of the power semiconductor device, a plurality of power semiconductor dies may be arranged in parallel in a single power semiconductor device. A semiconductor die is defined here as a die where a semiconductor circuit is formed on a Si crystal or SiC crystal. A semiconductor device is defined as a device in which a semiconductor die is incorporated to form a package.
FIG. 1 shows a schematic view of a cross section of a power semiconductor device 1. A power semiconductor die 2 has a metal electrode thereon. One end of a metal wire 5 is connected to the metal electrode, and the other end of the metal wire is connected to a lead frame of the power semiconductor device, an electrode on a substrate 6 or a metal electrode on another power semiconductor die among a plurality of power semiconductor dies being mounted to build a circuit. An intended metal electrode on the power semiconductor die 2 is referred to as a die electrode 3 and a metal die to be connected to the semiconductor die 3 is referred to as a connection electrode 4.
In a power semiconductor device, a heavy Al wire having a diameter of several hundred μm is used for the metal wire to have connection between electrodes (Patent Document 1). Although Cu wire is used for some LSIs as a metal wire, Cu wire is not used as a heavy wire for a power semiconductor device. Because Cu wire has high hardness and high work hardening coefficient, use of heavy wire likely damages a die electrode when a bonding connection is made by using ultrasonic waves. Au wire is frequently used for making ball bonding for LSI. However, as a heavy Au wire to be used for a power semiconductor device is very costly, it is hardly used for practical purposes.
Patent document 2 shows a bonding wire made of core material consisting of Ag or Ag alloy and an external coat thereon consisting of Au. Patent document 3 shows a bonding wire made of a core material containing Ag and a coating layer thereon containing Pd, Pt and/or Ni. However, the intended wire in both cases is 25 μm or not greater than 40 μm. Also as described in Patent document 2, Ag wire bonding with Al electrode causes significant amount of Al2Ag intermetallic compound to form. Because the intermetallic compound is so brittle that reliable bonding cannot be obtained, this prevents in particular Ag wire from being used for a power semiconductor device. Patent document 4 shows an example where Ag is used as a conductor for making a connection between an electrode and a pad in the power semiconductor module. However, the conductor and the pad and/or electrode are bonded with a solder, which is not intended to be applied to ball bonding and/or wedge bonding.
Patent document 5 also shows an example where an Au-based or Cu-based bonding wire connection or bonding ball is formed on an Au-based surface metal layer which is formed on a barrier metal film disposed on an Al-based or Cu-based bonding pad on a semiconductor chip which is a part of a semiconductor integrated circuit device. This prevents poor bonding, such as a Kirkendall void. However, this applies to Au wire or Cu wire bonding and is not intended for a power semiconductor device using a heavy wire.
When the power semiconductor device is modified to have a larger capacity, this leads to a large amount of current flow in a built-in power semiconductor die. When a large amount of current flows repeatedly in a power semiconductor die, the semiconductor die generates a lot of heat, and heat generated from a metal wire is considerable. This heat generation causes a temperature of the semiconductor die and the metal wire connected thereto to increase and decrease repeatedly. This causes thermal stress in a bonding part depending on the thermal expansion difference between members, and the thermal stress may cause a crack in the vicinity of the interface between the electrode and the metal wire.
In a recent power semiconductor device using a power semiconductor die with SiC substrate, it is said that an operation at high temperature of 250° C. or more can be made because the bandgap of SiC crystal is much larger than that of Si crystal. Thus, power semiconductor devices operable at high temperature have been studied. In such a power semiconductor device operable at high temperature, the thermal resistance of the device becomes more important than ever. As a melting point of conventional Al wire is relatively low, around 660° C., when it is used at high temperature, recrystallization occurs, which leads to forming a crack or void defect inside the wire and/or at an interface of the bonding part.